Meiosis is a fundamental process that sexually reproducing organisms undergo in order to reduce by half the chromosome number in germ cells. This reduction is necessary so that when two gametes fuse at fertilization the diploid chromosome number of the cell is reconstituted. When meiosis fails, chromosomally imbalanced gametes result. In mammals, the zygotes generated by fertilization of such chromosomally imbalanced gametes are frequently inviable and account for a large number of spontaneous abortions. In cases where viable offspring are produced, mental and morphological defects such as those seen for Trisomy 21 (Down's syndrome) are observed. Understanding how the meiotic process works to accurately segregate homologous chromosomes may ultimately provide the knowledge needed to monitor and prevent failures of the process. In order for homologous chromosomes to segregate properly at the first meiotic division, they must first become physically associated by formation of a multi-protein structure called the synaptonemal complex (SC). The focus of the proposed grant is to determine how the SC is assembled during meiosis and how it functions to direct the segregation of homologous chromosomes to opposite poles at Meiosis I in the yeast Saccharomyces cerevisiae. Two genes, RUM17 and RUM18, have been recently identified using a screen specific for genes involved in meiotic chromosome synapsis. Mutants in these genes exhibit reduced levels of interhomolog recombination and decreased spore viability. In addition, the mechanism by which an acyltransferase, HCS1-7, is able to specifically suppress a defect in the synaptonemal complex component HOPI, will be investigated. In order to find additional genes important for synapsis, two genetic screens, high copy suppression and synthetic spore lethality, will be performed using specially selected alleles of HOP1, RED1, MEK1 and HCS1-7. Mutations in genes arising from the genetic screens will analyzed for their effects on SC formation, recombination and chromosome segregation to assess their functions. Biochemical experiments using a combination of purified proteins and extracts will be performed to determine whether gene products which interact genetically do so by a direct physical interaction. Specifically the model that RED1 physically interacts with HOP1, as well as the model that HOPI is a substrate for the MEK1 kinase will be tested. By combining genetic, biochemical and cytological studies, a greater understanding of the function, structure and pathway of assembly of the SC will be obtained.